This invention relates to thermosetting resin compositions, in which shrinkage is reduced by the use of thermosetting polymeric additives.
It is well known that when monomers are polymerized, a volume reduction occurs. This shrinkage causes changes in dimensions, internal stresses in the cast article, cracking and/or separation from the mold wall yielding surfaces of inadequate smoothness. Thermoplastic additives are often incorporated into polyester resins in order to reduce shrinkage on curing of these resins. See for example (Fritz M. Wright--New "No Shrink" Polyester Resin for BMC, SMC and Wet Moulding--Paper 12A, 27th Annual Conference Proceedings. . . Reinforced Plastics/Composites Institute (1972)).
Thermosetting additives are also known; U.S. Pat. No. 4,104,241 teaches the use of 2-hydroxyethyl acrylate adducts of poly(styrene co maleic anhydride) as reactive polymeric additives. The preparation of the above additives is a two stage process, it is an object of the present invention to provide compositions for which the polymeric additive is prepared by a single stage process. Such unsaturated polymers as those used in the present composition have been prepared previously (U.S. Pat. No. 4,059,616). However anionic polymerization was employed rather than the free radical type polymerizations preferred here. U.S. Pat. No. 4,059,616 states that free radical polymerization does not yield useful polymers.